It is a requirement from the customers for a video camera or/and digital camera to have a compact size and high quality of resolution. A zoom lens assembly is therefore a basic requirement for the products. If the main beam of light coming from the object is not perpendicular to the lens assembly, for the cambers that employ a square pixel to form an image, can only obtain an image that has low illumination on the periphery, the color changed slightly and/or vignetting of the image simply by using the zoom lens assembly and the efficient of CCD. This is the main factor that makes the lens assembly to be complicated. It is a goal for the manufacturers to have a tele-centric that has a parallel beam of light with the light axis when the light comes from the object.
The cameras that use the charge coupled device (CCD) and a low pass filter lens or color compensation filter lens is installed between the rear end of the lens assembly and the CCD so as to prevent astigmatism of the image and reduce the deformation of the colors of the light sensing feature of the CCD. The back focal length (BFL) for these lens assembly is longer than the conventional so that the above feature is necessary. Conventional zoom lens assembly having the feature includes a first group of lens that has negative power and a second group of lens that has positive power and a third group of lens that has positive power. This technology may zoom in and out up to 2 to 3 times of the ratio of the image.
Japanese published patent H3-288113 tries to improve the shortcoming of the conventional lens assembly and makes the first group of lens to be a fixed one. In order to let the image to be focusing, the diameter of the first lens has to be increased so that the number of the lens is increased and the tele-centric is increased.
Another Japanese published patent H10-104518 includes three groups of lens that are negative power, negative power and positive power. The second group is made of four lenses of positive, gluing, and positive. This allows the lens assembly to have three times of zoom-in and zoom-out ability. Nevertheless, there are so many problems such as the diameters of the lens, the length of the tele-centric and the sensitivity of de-center of the second group of lens, to be settled when in mass production.
Japanese published patent 2000-8911 discloses a lens assembly composed of three lenses which are negative, negative and positive. The second group of lens is composed of three lenses which are positive, negative, and positive. Although the number of the lenses is reduced, the diameter and the character of the de-center are not improved.
Japanese published patent 2001-318311 discloses a lens assembly composed of three lenses which are negative, negative and positive. The second group of lens is composed of three lenses which are positive, glued negative and positive. Although the length of the tele-centric is shortened and the character of the de-center is not improved, the diameter cannot be reduced.
Japanese published patent 2001-281545 discloses a lens assembly composed of two lenses wherein the first group of lens comprises two lenses which are a negative and a positive lens, and the second group of lens is composed of four lenses which are glued positive and negative lens, and glued negative and positive. Although slim of the first group of lens reduces the diameter of the lens assembly and the length of the tele-centric, the number of lens for the second group of lens is not satisfied.